I find it very easy to tell the differance.

That is close to printing an 8MP image at 8x12 vs. 12x18, the 12x18
prints are clearly getting  soft.

But then I am pretty near sighted and so tend to see things many others
don't.

Scott

Can you differentiate between...people??

Many will never notice.  Others will...with some caring and others not.
Much of it depends on the nature of the subject.
Images with small, interesting details in the subject will cause difficulty,
while less-complex subjects will look comparatively great.  These are just
examples of why there is no hard and fast "rule" for printing dpi.

As a former large format printer a couple things to note first I used
Epson printers and they tend to like their ppi to be divided by 60. So
between 200 and even 240ppi there is a difference especially on a
smaller print like a 12x18. Beween 240ppi and 300ppi less of a
difference, but between 240 and 360ppi there is a noticeable
difference. I worked mostly with 6mp files from Kodak DCS760s and
scanned 35mm and mf files.

Tom

It depends on the subject matter.

My eyesight is extremely good. For me, spotting the difference between
200 & 300 DPI depends a lot on the medium, the image, how the image
was processed, & whether I have the full-res original onscreen to
compare to the print. For most people, most of the time, the
difference is fairly academic.

From a comfortable viewing distance, i.e. taking in the complete
picture, you would be hard pressed to see any difference.

Pixel-peepers who leave nose smears on the print will claim they can see
a difference - but they're not looking at the picture.

Reminds me of the story about a hi-fi fanatic who was always complaining
to the store about how his system sounded.  Finally, the salesman said
"Quit listening to your amplifier, and instead listen to the music" The
moral applies to photogs as well.

Colin D.

At normal viewing distance, not many can.  Print up two the same end
size at both printer resolutions and see what _you_ think.

Assuming that the 300 dpi source is not intentionally
blurred, the answer is a resounding yes, they can tell. I'm
assuming here that you have say two 8x10 prints, one from
a 1600x2000 pixel source and one from a 2400x3000 source,
and a suitable test subject, like a photo of a printed book,
or a scenic picture all at infinity with lots of detail. Now
if the photo is a portrait with a narrow depth of field,
then the average person may very well see no difference. The
portrait pro may look at the eyes and see a difference,
or maybe even they won't, depending ....

Now 300 dpi to 500 dpi is much harder to tell apart, but for
flat macro subjects or scenics, even there the difference
is present, if the originals are sharp. A good subject
here would be to compare 300 and 500 dpi scans of a
low-speed Fuji positive film 8x10 original transparency
of a scenic, or a scan of a printed page from a fancy
medieval manuscript.

Doug McDonald